A number of occasions that destroy the ecosystem of the earth, such as acid rain and photochemical smog, are always caused by excessive emission of NOx, which is one of the main factors of air pollution. Therefore, the control of NOx emission becomes a key and difficult problem of atmospheric environment protection in the world. In China, for example, the NOx emission from combustion of fossil fuels, such as coal, petroleum and natural gas, takes up 90% of the total NOx emissions. Among them, 70% of the NOx emission is caused by coal combustion, which mainly comes from the power plant, industrial boiler and kiln. However, the control measures to these emissions, such as NOx from power plants and industrial boilers, are not mature enough. Meanwhile, lots of countries have established rigorous NOx emission standards to control the NOx concentration in atmosphere. For example, in China, the “Atmospheric Pollutants of Power Plant Emission Standard” (GB-13223-2003), which was applied to power plants with scale above 65 t/h of steam production capacity, confined the maximum acceptable emission concentration of NOx in flue gas. The NOx emission reduction has also been referred to in the agenda of “the 12th Five-Year plan” of China. Therefore, the denitration (DeNOx) of flue gas will be the key field in the atmospheric pollution control in future.
At present, the selective catalytic reduction (SCR) of NOx by NH3 is one of the most important methods to decrease the NOx emission of flue gas, to be specific, 95% of flue gas is processed by this method. The main NH3—SCR reactions can be illustrated as follows.4NH3+4NO+O2=4N2+6H2O  (1)4NH3+2NO2+O2=7N2+6H2O  (2)
The core of this technology is to develop DeNOx catalyst with high performance. In China, the production technology for the so-called medium-temperature DeNOx catalyst is originally from technical import and in turn self-development. The catalysts are based on the vanadium-tungsten-titanium oxides compound, which takes up 80% in weight. Therefore, the price of DeNOx catalysts is high.
Besides power plants, there are a large number of industrial combustion furnaces in China, including more than 500 thousands of industrial boilers consuming more than 400 million tons of coal per year, 180 thousands of calcination furnaces and rotary kilns consuming 0.16 billion tons of coal per year, and thousands of sintering furnaces with coal consumption of 70 million tons per year. However, the control of NOx in flue gas from these industrial furnaces is different from that from power plants. The capacity of industrial boilers is often smaller than 100 MW so that the cost of DeNOx system is more expensive per unit. For example, the cost of DeNOx catalyst for a heating boiler of 75 MW is usually about 200 RMB/h for reducing its flue gas NOx concentration to 50 mg/Nm3. Meanwhile, for these industrial boilers, the temperature window between air preheater and economizer is always in the range of 250-350° C., which is lower than that of power plants. Therefore, the conventional DeNOx catalyst is hardly adapted to the flue gas conditions of such industrial furnaces. On other hand, a large portion of industrial furnaces employ fixed bed, moving bed or stoker boiler, which have low dust in their flue gas (<10 g/m3). With growing environmental concern, the emission standard of NOx for industrial boilers should be more stringent in the future.
Given the preceding special flue gas conditions of China, there is a strong need for cheap DeNOx catalyst with wide working temperature window because they can be applied to the flue gases of both power plants and industrial boilers.
Since the 1990s, various DeNOx catalysts, including noble metal catalysts, transition metal oxide catalysts, zeolite and active carbon catalysts have been reported in the literature. The commercialized catalyst eventually adopted manganese and vanadium oxides as its active components. The DeNOx catalysts invented by Chinese patents 200910077934.4, 200910087773, 200810020426.8 and US patents US 20080072575 A1 and US 20100284876 A1 showed good low temperature activity. However, they are prone to be deactivated in the existence of SO2 and H2O. Cheap materials were also used as the base material of DeNOx catalysts. The Chinese patents 201010100867.6, 201010523591.2 and 200810112624.7 proposed the use of slag as additive in extruding honeycomb DeNOx catalysts. Yang R. T. invented a type of catalyst using TiO2 pillared bentonite to prepare cheap DeNOx catalyst (U.S. Pat. No. 6,521,559). The catalyst with 5 wt. % vanadium oxide shows good activity at 350° C. However, these catalysts hardly ensure the close interaction between active component and support, resulting in the low catalytic activity and high price of the catalysts.